The present disclosure is directed to a system for making several charged species by a pulsed DC spark discharge acting on an inert gas, typically helium, which utilizes the charged species to classify and/or quantify compounds in a gas sample. This detector is connected with upstream or downstream devices such as a sample source, gas chromatography (GC) column, spectrum analyzers, etc. Understanding of various test procedures will illuminate use of the described apparatus and can be gained from review of the apparatus and its mode of operation in a system. A sample to be evaluated is first loaded along with a carrier gas into a system column. The sample passes through this device, a pulsed, high voltage discharge, and several types of detection systems are initiated by this detector. For instance, the very short DC spark creates a readily available thermalized electron flux which can be used in a detection system. In an alternate mode of operation, the spark also creates a more slowly diffused flux of metastable helium atoms which drift toward into a gas sample at a controlled rate. The helium atoms will react with molecules of the gas sample to surrender the excess energy from the excited state to cause sample molecule ionization which, as a secondary reaction, can be measured by a detection system. Another aspect involves transitory photo ionization of a gas into positive and negative charged particles normally recombining at high speed. If a selected bias voltage is applied, the recombination is prevented to furnish a current signal indicative of gas contents.
The preferred form of this system features a pulsed DC spark discharge in the inert gas flow which is followed by a comparably slow metastable carrier gas dispersion and secondary reaction, which is slow in contrast with the practically instantaneous electron initiated interaction the time of the spark. The DC spark discharge therefore enables various detection mechanisms, as will be explained, so that variations in detection electrode geometry and pulse timing can obtain different types of responses. One system uses the highly mobile electron flux while an alternate system relies on the metastable carrier gas molecular energy interchange occurring well after the electron flux. An electron capture detector is set forth. Also, an air monitor is disclosed.
In addition to the particle interaction initiated in the spark manifest in different aspects, there are also two electrode systems responsive to the DC spark. From the spark gap, the electron discharge creates charged species which can be observed at spaced electrodes. Geometry of the spark is sharply defined, narrowly confined, and repetitively located.
This device enables detection of the atomic species in the gas sample. While a first spectrum is formed only during the spark, a second spectral analysis is enabled by the subsequent decay of the metastable helium atoms giving up their excess energy by ionizing molecules of the sample. This interchange occurs as the energized helium atoms diffuse from the spark gap in the test chamber and mix with the sample molecules. Dependent on relative concentrations, diffusion and flow rates, the sample molecules are ionized to emit energy characteristic of the species. This delayed emission is useful in species identification when timely observed, and therefore a different mode of observation is used capture data from this emission. This difference in operation derives primarily from delayed occurrence and is observed at a different time.
The present invention uses to advantage a simple spark gap having a pair of spaced electrodes connected to a current pulse forming system. The pulses are narrow, preferably as small as a fraction of a microsecond. The DC pulses repetitively form precise, sharp and well defined transgap sparks, liberating the electron flux mentioned and also forming the metastable helium molecules. The spark is fixed in size and relative timing, shape and location. Electrode geometry does not erode with time and electron ejection is uniform. Thus, the spark is fixed for observation by spectral analysis. Structurally, this enables a very simple chamber to deploy a pair of opposing, spaced electrodes in a cavity of small volume with gas flow inlet and outlet ports. In a representative system, a chemical sample is mixed with a carrier gas. The sample is prepared for testing by classification, identification or quantification using the detector. An exemplary system achieves separation as a result of differences in travel time through a GC column input to the detector. As is well known, the GC column is either a wall coated open capillary or packed with a stationary phase material so that the carrier gas and the compounds making up the sample are eluated from the GC column. As a generalization, the mobile phase (usually a flowing gas) is delivered by the GC column into this detector for detection of the separated chemical constituents making up the sample.
The detector is operated periodically to test every sample constituent compound passing through the detector. One type of detector used in the past has been the electron capture detector (ECD). The present disclosure sets out an alternate form of ECD detector used in conjunction with a GC column which forms an output signal of substantial sensitivity. The present system features an ECD with a DC pulsed, high voltage spark discharge. As noted at column 2 of U.S. Pat. No. 4,851,683, DC discharges have been known, but they generally have had in homogeneous excitation characteristics and are unstable because of electrode heating and erosion. U.S. Pat. No. 4,509,855 is a DC atmospheric pressure helium plasma emission spectrometer. Additional devices are shown in U.S. Pat. No. 4,866,278. The present apparatus sets forth a DC pulsed, high voltage, spark discharge source which provides a repetitive uniform spark. The spark has a duration which is only a fraction of a microsecond. It would appear theft an acceptable spark duration is a fraction of a microsecond. Moreover, the spark gap is structurally fixed to have a finite width for discharge of the spark created by accumulating energy in a reactive circuit such as a coil and capacitor charging. Preferably, a non-ringing current is applied.
This detector in a representative form includes a means for forming a stabilized spark gap so that the spark and resultant charged particle population are uniform in contrast with the problems referenced in the two mentioned patents. Accordingly, the carrier gas (e.g., carrier flow from the GC column) is directed as a gas flow through appropriate tubing into the spark chamber. An inert gas flows in the spark chamber past a pair of electrodes which are arranged to direct the spark transverse to the inert gas flow. In a first mode of operation, a flux of electrons is obtained. These electrons are quickly dissipated during the spark interval even when spark duration is only a fraction of a microsecond. The number of electrons available can be measured by means of an electrometer connected to electrodes spaced remotely from the spark gap. The electrometer circuitry connected with an electrode in the chamber and spaced from the spark gap detects and measures the electron flux resulting from the spark discharge. In this instance, the spark gas initiates an ECD operation. There is, however, a timed charged particle flux which is delayed after the spark discharge which uses an ionization mode. This involves a delay of up to about 100 or even 200 microseconds after the spark discharge creates ionized molecules which are dispersed at a slower rate compared with the more mobile electron dispersal. The spark disperses highly energized electrons during the spark and also creates a second and slower dispersion of metastable inert gas molecules (preferably helium) after the spark. Charged particle dispersal of the first form is, as a practical matter, instantaneous while metastable helium dispersal is time delayed. The two types of dispersion are readily identified because they involve different types of particles. The dispersal of metastable helium atoms, with an elevated energy state of about twenty or more eV, can be observed at a distance from the spark gap so that sample compound concentration (a scale factor) in the chamber is measured. The metastable helium concentration is useful because it enables this delayed reactions. Thus, the metastable helium atoms react with the sample molecules input with the carrier flow. The high energy in the helium ionizes the sample molecules, creating a measurable current in the chamber.
Building on the last possibility, metastable helium molecules may combine with a trace constituent such as a dopant supplied with t:he inert (helium) gas. One such dopant is nitrogen which, in reaction with the metastable helium, forms nitrogen ions. That causes liberation of electrons which again, because of different mobility, dissipate more readily. Before the electrons recombine with the ionized nitrogen molecules, they will react with the compounds making up the sample flowing through the detector. A connected electrode and electrometer measures electron capture from the dopant involvement to define an electron capture detector.
Another alternate form of apparatus involves observation of the spectrum. This involves the conversion of the certain constituents to elevated energy states where emissions occur at characteristic frequencies, and such frequencies can be observed and measured. This typically involves a spectrum analyzer such as a spectrometer which observes one or more atomic or molecular emission lines in selected regions of the spectrum. Spectral line observation varies with the time relative to the spark discharge. Regarding time, the observed spectrum is different during and after the spark discharge. The present apparatus is therefore summarized as a pulsed DC spark discharge where the spark discharge reacts with an inert gas (preferably helium) to detect compounds in a sample. In this spark, charged particles are created, the particles being either disassociated electrons, an ionized inert gas, ionized dopant gas, or highly energized helium atoms in a metastable form. Depending on the timing of measurements, the particular ionized particles and measurement voltages applied, the device can be operated in an ionization mode, or electron capture mode. Molecules of a compound separated by chromatographic separation or other input devices can be quantified. The device also emits characteristic spectral lines depending on the nature and timing of the emission. Moreover, by selection of the dopant gas, control of pulsing of the spark gap forming the charged particles, timed operation of measurement electrodes, and adjustment of scale factors, it is possible to operate in several modes. In addition to this, precisely defined spectral lines can be observed.
The present apparatus additionally includes simplified versions of the pulse discharge mechanism cooperative with a GC system. In one instance, the helium metastable molecule is used to achieve ionization of the eluate from the GC column without forcing the eluate to flow through the spark gap. This enhances operation of the equipment because the spark acts primarily on helium, while the electrodes are protected from contamination by the solvent or the eluate sample flowing from the GC column. In this version of equipment, the GC column discharge is delivered into the chamber at a location where it is not required to flow through the spark gap. As a second alternative, a dopant gas is input to the detector. Further, this type arrangement enables the system to operate as a simple ionization detector. Alternately, it can be operated as an electron capture detector (ECD hereafter). Details of these structures will be given later. Another aspect of the present apparatus is the use of the device to form an emission spectra which provides spectra from various samples through a transparent window. In this aspect of the system, it is provided with a transparent window sealed at the entrance of a monochromator. In this aspect of the invention, the helium gas flow plus the eluate from the GC column is across the transparent window so that the reaction products do not contaminate the window which loses transparency as a result of impinging contamination. So to speak, the window is located to view the mixing. Through the use of this mechanism spectral emissions can be obtained to analyze the constituent components of a sample. For instance, characteristic atomic, ionic, or molecular spectra lines can be classified. One characteristic of the atomic spectra is formation of extremely narrow emission lines with little or no interference between spectra from other atoms or molecules. This is especially helpful in the vacuum ultraviolet region. By contrast, the ultraviolet and visible regions of the spectra may receive broad interfering spectra from many common elements or molecules. Accordingly, it is especially desirable to operate in the vacuum ultraviolet region and in particular the region of about 120-200 nanometers.